The development of semiconductor switching technology for high power applications in motor drive circuits, appliance controls, robotics and lighting ballasts, for example, began with the bipolar junction transistor. As the technology matured, bipolar devices became capable of handling large current densities in the range of 40-50 A/cm.sup.2, with blocking voltages of 600 V.
Despite the attractive power ratings achieved by bipolar transistors, there exist several fundamental drawbacks to the suitability of bipolar transistors for all high power applications. First of all, bipolar transistors are current controlled devices. For example, a large control current into the base, typically one fifth to one tenth of the collector current, is required to maintain the device in an operating mode. Even larger base currents, however, are required for high speed forced turn-off. These characteristics make the base drive circuitry complex and expensive. The bipolar transistor is also vulnerable to breakdown if a high current and high voltage are simultaneously applied to the device, as commonly required in inductive power circuit applications, for example. Furthermore, it is difficult to parallel connect these devices since current diversion to a single device occurs at high temperatures, making emitter ballasting schemes necessary.
The power MOSFET was developed to address this base drive problem. In a power MOSFET, a gate electrode bias is applied for turn-on and turn-off control. Turn-on occurs when a conductive channel is formed between the MOSFET's source and drain regions under an appropriate gate bias. The gate electrode is separated from the device's channel by an intervening insulator, typically silicon dioxide. Because the gate is insulated from the channel, little if any gate current is required in either the on-state or off-state. The gate current is also kept small during switching because the gate forms a capacitor with the device's active area. Thus, only charging and discharging current ("displacement current") is required. The high input impedance of the gate, caused by the insulator, is a primary feature of the power MOSFET. Moreover, because of the minimal current demands on the gate, the gate drive circuitry can be easily implemented on a single chip. As compared to bipolar technology, the simple gate control provides for a large reduction in cost and a significant improvement in reliability.
These benefits are offset, however, by the relatively high on-resistance of the MOSFET's channel, which arises from the absence of minority carrier transport. As a result, the device's operating forward current density is limited to relatively low values, typically in the range of 10 A/cm.sup.2, for a 600 V device, as compared to 40-50 A/cm.sup.2 for the bipolar transistor.
On the basis of these features of power bipolar transistors and MOSFET devices, hybrid devices embodying a combination of bipolar current conduction with gate-controlled current flow were developed and found to provide significant advantages over single technologies employing bipolar junction transistors or MOSFETs alone. Thus, in the insulated gate transistor (IGT), disclosed in an article by coinventor B. J. Baliga, M. S. Adler, R. P. Love, P. V. Gray and N. Zommer, entitled "The Insulated Gate Transistor: A New Three terminal MOS Controlled Bipolar Power Device," IEEE Trans. Electron Devices, ED-31, pp. 821-828 (1984), on-state losses were shown to be greatly reduced when compared to power MOSFETs. This was caused by the conductivity modulation of the IGT's drift region during the on-state. Moreover, very high conduction current densities in the range of 200-300 A/cm.sup.2 can also be achieved. Accordingly, an IGT can be expected to have a conduction current density approximately 20 times that of a power MOSFET and five (5) times that of an equivalently sized bipolar transistor. Typical turn-off times for the IGT can be expected to be in the range of 10-50 .mu.s. A cross-sectional representation of a typical insulated gate transistor is shown in FIG. 1.
Although gate-controlled transistors such as the IGT represent an improvement over using bipolar or MOSFET devices alone, even lower conduction losses can be expected by using a thyristor. This is because thyristors offer a higher degree of conductivity modulation and a lower forward voltage drop as compared to the IGT. Consequently, the investigation of thyristors is of great interest so long as adequate methods for providing forced gate turn-off can also be developed. As will be understood by one skilled in the art, a thyristor in its simplest form comprises a four-layer P1-N1-P2-N2 device with three P-N junctions in series: J1, J2, and J3, respectively. The four layers correspond to the anode (P1), the first base region (N1), the second base or P-base region (P2) and the cathode (N2), respectively. In the forward blocking state, the anode is biased positive with respect to the cathode and junctions J1 and J3 are forward biased and J2 is reversed-biased and most of the forward voltage drop occurs across the central junction J2. In the forward conducting state, all three junctions are forward biased and the voltage drop across the device is very low and approximately equal to the voltage drop across a single forward biased P-N junction.
An inherent limitation to the use of thyristors for high current applications is sustained latch-up, however, arising from the coupled P1-N1-P2 and N1-P2-N2 bipolar transistors which make up the four layers of the thyristor. This is because sustained thyristor latch-up can result in catastrophic device failure if the latched-up current is not otherwise sufficiently controlled by external circuitry or by reversing the anode potential. Sustained latch-up can occur, for example, when the summation of the current gains for the thyristor's regeneratively coupled P1-N1-P2 and wide base P1-N2-P2 transistors exceeds unity. An alternative to providing external circuitry or reversing the anode potential to obtain turn-off, however, is to use a MOS-gate or similar device for controlling turn-on and turn-off.
Several methods for obtaining gate control over thyristor action exist. For example, in the MOS-controlled thyristor (MCT), turn-off is provided by shorting the emitter-base junction of the N-P-N transistor to thereby produce a reduction in gain. This form of control ideally raises the holding current of the thyristor to a level above the operating current level. Accordingly, an MCT structure has been reported which utilizes a P-channel MOSFET integrated into the cathode region of a thyristor for turn-off control and an N-channel MOSFET integrated into the P-base region for turn-on control. This device and its complementary counterpart are described in an article by V. A. K. Temple, entitled "The MOS Controlled Thyristor," published in IEDM Technology Digest, Abstract 10.7, pp. 282-285, (1984). FIG. 2 schematically illustrates a prior art MCT and is a reproduction of FIGS. 2 and 3 from the aforesaid Temple article. Although gate-controlled conduction is possible with the MCT, the maximum controllable current density, which is a direct measure of a device's ability to turn-off, is limited by the MOSFET inversion-layer channel resistance and other resistances in the base region. Because of the lower mobility for holes in silicon, MCTs built from n-type high-voltage drift layers exhibit poor current turn-off characteristics. MCTs, such as the one described in U.S. Pat. No. 5,105,244 to Bauer, have also been built to include inverted diodes between the anode and cathode for providing enhanced inductive-load switching characteristics, without the need for an external diode in antiparallel with the MCT.
Other examples of MOS-gated thyristors include the depletion-mode thyristor (DMT), shown in FIG. 3, which overcame many of the drawbacks associated with the MCT. In the DMT, a depletion-mode MOSFET is placed in series with the base of the P-N-P transistor. Accordingly, once the thyristor is turned-on, current flow can be shut off by application of a negative gate bias. This eliminates the base drive by pinching off the base current to the P-N-P transistor and shuts off the device. Also, in U.S. Pat. No. 5,144,401 to Ogura et al., an insulated-gate thyristor is disclosed having two gate electrodes for controlling turn-on and turn-off. The first gate is separately isolated from the first base region and operates a MOSFET between the second emitter region (cathode region) and the first base region. The second gate is laterally disposed with respect to the cathode and electrically contacts the second base region. Both turn-on and turn-off require the sequential control of the two gate electrodes.
Recently, a base resistance controlled thyristor (BRT) was described in U.S. Pat. No. 5,099,300, to inventor B. J. Baliga, and an article entitled "A New MOS-Gated Power Thyristor Structure with Turn-Off Achieved by Controlling the Base Resistance," by M. Nandakumar, inventor B. J. Baliga, M. Shekar, S. Tandon and A. Reisman, IEEE Electron Device Letters, Vol. 12, No. 5, pp. 227-229, May, 1991, both of which are hereby incorporated herein by reference. The BRT operates by modulating the P-base resistance of the thyristor using MOS-gated control. Operational BRTs with 600-volt forward blocking capability, such as the one shown in FIG. 4, have been developed. FIG. 4 is a reproduction of FIG. 1 from the aforesaid Nandakumar, et al. article. The BRT can be turned-off by the application of a negative bias to a P-channel enhancement-mode MOSFET to thereby reduce the resistance of the P-base by shunting majority charge carriers from the P-base to the cathode. As will be understood by one skilled in the art, the reduction in P-base resistance results in an increase in the device's holding current to above the operational current level and shuts-off the device. Like the reported MCT structure, the BRT requires dual-polarity gate control for operation.
It would be preferable to provide a switching device operable with single-polarity gate control and without the limitation associated with the reported MCT. Moreover, it would also be preferable to have a structure capable of being processed with relatively few process steps, using relatively few masks and capable of being highly integrated in a semiconductor substrate.
Some of these preferred features can be found in U.S. Pat. No. 5,014,102, issued to Adler, entitled Mosfet-Gated Bipolar Transistors and Thyristors with Both Turn-On and Turn-on Capability Having Single-Polarity Gate Input Signal. This patent discloses a triple-diffused MCT structure with separate enhancement and depletion-mode MOSFETs for providing turn-on and turn-off control, respectively. Turn-off control is provided by a separate termination region in the cathode region and by a P-channel depletion-mode MOSFET between the second base region and the termination region. The termination region is electrically connected to the cathode contact.
Unfortunately, because the termination region is formed by diffusion into the cathode region, the cathode region is large in terms of its lateral dimensions, so that high integration densities may be difficult to achieve. Moreover, the fabrication process described for the Adler MCT requires a relatively costly triple-diffusion fabrication process when compared to the process for forming a BRT, which is basically an IGT baseline process, as reported in the above-referenced article on the BRT. Another consequence of the cathode region being relatively large is the size of the second base region (P-region 174 in Adler) which is correspondingly large because the cathode region is in the second base region. As will be understood by one skilled in the art, the relatively large size of the second base region causes a respective increase in the majority carrier (charge) concentration in the second base when the thyristor is conducting and limits the maximum controllable current density, i.e., the range of on-state current values that can be turned off when a proper gate bias is applied.
A base resistance controlled thyristor (BRT) with single-polarity control and high forward voltage blocking capability is described in copending application entitled A Base Resistance Controlled Thyristor With Single-Polarity and Dual-Polarity Turn-On and Turn-Off Control, Ser. No. 07/919,161, filed Jul. 23, 1992, by inventor B. J. Baliga. This invention includes means for diverting current from the second base region to the cathode via an adjacent region in the substrate. To provide single-polarity control, a "normally-on" channel comprising a doped region in the first base region is provided to shunt majority carriers from the second base region to the cathode, via the adjacent region, and turn-off the thyristor in the absence of a turn-on gate bias.
Notwithstanding the improvements obtained by the invention of the above-noted Baliga application, it would be advantageous to provide a single-polarity controlled thyristor without the need for an adjacent region for shunting current to the cathode. For example, the elimination of the adjacent region for shunting current away from the second base region may provide a substantial reduction in the lateral dimensions of the unit thyristor cell. Higher integration densities may also be produced, resulting in higher total current densities when multiple unit cells are connected in parallel.